Star-delta connection

I have read that on generating side there is star delta connection and delta-star connection on distribution side..>!!
I wanted to know the reason behind it..!!
I know in delta the line voltage =phase voltage & in star phase voltage reduces..so is this the part of the reason behind it..??
Please elaborate..!!

Not quite clear to what you are referring to -with the "delta-star connection on distribution side" but typically the Star connection ( with a neutral) is on the Source side of the network connection making it the "secondary" side of the transformer depending on the direction of power flow. This is done for protection and safety (grounding ) purposes - so that the loads have a stable and ground referenced voltage.
Otherwise if you can post a link to what you are reading it may help clarify.

sorry , I did a mistake , actually our prof. taught us that on genrating side of power plant the transformer is in delta-star configuration where as on distribution side it is in star-delta config. , the reason being that voltage should be kept lower..!! but why its in the stated config. why not star-delta onbboth transmission & distribution side??
I did'nt get this.. this is what I want to get elaborated..

You should earth any electrical power source and you should do that where its current originates.

The generator is earthed at its own neutral, so there's no need to have a neutral on the stepup transformer's primary because it sees earth via the generator..
The stepup's secondary becomes the origin of current for distribution system, so we make that a star winding and earth its neutral.

In my opinion, that depends on application. If the secondary is connected to medium voltage overhead line then secondary has to be delta.

The medium voltage overhead lines [and even cables] could work for a while in a phase-to-ground short-circuit and there is no interruption of supplied energy.

High-voltage neutral is grounded [or nearly] in order to reduce voltage to ground in secondary winding [the stress between grounded elements and the winding] and the stress in overhead line insulators.

For this reason high voltage side- the secondary winding in step-up transformer- is star.

Reduction of harmonics content and influence of asymmetries to the sync. generator's work is better achieved in Y-D schemes. If the generator's stator is wye and is loaded by a nonlinear, but ballanced delta connected load, line currents don't contain triplen harmonics and DC component. These harmonics and DC components still circulate around delta. However, in practice loads are both nonlinear and unballaced which makes content of harmonics to appear in line currents. Triplen harmonics (3rd,6th,...) are quite troublesome, they can excite various bad modes in the generator. Some DYx transformer's groups are particularly good in suppressing influence and presence of most dangeorous harmonics. Therefore, it's not just "gen's wye - xfmr's delta", but whole "Y-DYx" thing that does the job. Another reason why we frequently see such connections in power transmision schemes is money. It's less expensive to construct power xfmr with medium voltage windings in delta and high voltage windings in wye.